compilers: clang: map -Db_vscrt to -fms-runtime-lib

The main complication here is that passing -fms-runtime-lib during compilation
results in a warning:

   clang: error: argument unused during compilation: '-fms-runtime-lib=dll' [-Werror,-Wunused-command-line-argument]

(https://github.com/mesonbuild/meson/actions/runs/17727020048/job/50369771571).

So, for compilation expand the -D flags by hand, and only pass -fms-runtime-lib
when linking.

Fixes: #14571
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 39 insertions, 1 deletions

diff --git a/docs/markdown/snippets/b_msvcrt_clang.md b/docs/markdown/snippets/b_msvcrt_clang.md
new file mode 100644
index 000000000..2a509bda6
--- /dev/null
+++ b/docs/markdown/snippets/b_msvcrt_clang.md
@@ -0,0 +1,4 @@
+## `-Db_msvcrt` on clang
+
+`-Db_msvcrt` will now link the appropriate runtime library, and set
+the appropriate preprocessor symbols, also when the compiler is clang.
diff --git a/mesonbuild/compilers/mixins/clang.py b/mesonbuild/compilers/mixins/clang.py
index 72b987a12..863d2502d 100644
--- a/mesonbuild/compilers/mixins/clang.py
+++ b/mesonbuild/compilers/mixins/clang.py
@@ -54,6 +54,23 @@ class ClangCompiler(GnuLikeCompiler):
 
     id = 'clang'
 
+    # -fms-runtime-lib is not supported together with -c, emulate it
+    CRT_D_ARGS: T.Dict[str, T.List[str]] = {
+        'none': [],
+        'md': ['-D_MT', '-D_DLL'],
+        'mdd': ['-D_MT', '-D_DLL', '-D_DEBUG'],
+        'mt': ['-D_MT'],
+        'mtd': ['-D_MT', '-D_DEBUG'],
+    }
+
+    CRT_ARGS: T.Dict[str, T.List[str]] = {
+        'none': [],
+        'md': ['-fms-runtime-lib=dll'],
+        'mdd': ['-fms-runtime-lib=dll_dbg'],
+        'mt': ['-fms-runtime-lib=static'],
+        'mtd': ['-fms-runtime-lib=static_dbg'],
+    }
+
     def __init__(self, defines: T.Optional[T.Dict[str, str]]):
         super().__init__()
         self.defines = defines or {}
@@ -65,11 +82,19 @@ class ClangCompiler(GnuLikeCompiler):
         # linkers don't have base_options.
         if isinstance(self.linker, AppleDynamicLinker):
             self.base_options.add(OptionKey('b_bitcode'))
-        elif isinstance(self.linker, MSVCDynamicLinker):
+        elif isinstance(self.linker, MSVCDynamicLinker) or self.info.is_windows():
             self.base_options.add(OptionKey('b_vscrt'))
         # All Clang backends can also do LLVM IR
         self.can_compile_suffixes.add('ll')
 
+    def get_crt_compile_args(self, crt_val: str, buildtype: str) -> T.List[str]:
+        crt_val = self.get_crt_val(crt_val, buildtype)
+        return self.CRT_D_ARGS[crt_val]
+
+    def get_crt_link_args(self, crt_val: str, buildtype: str) -> T.List[str]:
+        crt_val = self.get_crt_val(crt_val, buildtype)
+        return self.CRT_ARGS[crt_val]
+
     def get_colorout_args(self, colortype: str) -> T.List[str]:
         return clang_color_args[colortype][:]
 
diff --git a/unittests/windowstests.py b/unittests/windowstests.py
index 2cb1407c1..bf1225d18 100644
--- a/unittests/windowstests.py
+++ b/unittests/windowstests.py
@@ -398,7 +398,16 @@ class WindowsTests(BasePlatformTests):
         if OptionKey('b_vscrt') not in cc.base_options:
             raise SkipTest('Compiler does not support setting the VS CRT')
 
+        MSVCRT_MAP = {
+            '/MD': '-fms-runtime-lib=dll',
+            '/MDd': '-fms-runtime-lib=dll_dbg',
+            '/MT': '-fms-runtime-lib=static',
+            '/MTd': '-fms-runtime-lib=static_dbg',
+        }
+
         def sanitycheck_vscrt(vscrt):
+            if cc.get_argument_syntax() != 'msvc':
+                vscrt = MSVCRT_MAP[vscrt]
             checks = self.get_meson_log_sanitychecks()
             self.assertGreater(len(checks), 0)
             for check in checks: